The long-term objectives of this research project are (1) to determine the effects of three interventions on physical activity and other health outcomes at 12 months post-randomization among 300 sedentary and underactive older adults aged 55 to 80 years, and (2) to determine whether prosocial behavior is a construct that can be successfully implemented within Social Cognitive Theory. Based upon our preliminary study, we have designed a prosocial behavior physical activity intervention that will incorporate opportunities to aid the Second Harvest Food Bank of Northwest North Carolina based upon weekly physical activity. Along with the established benefits associated with physical activity, the public health relevance of this project relates to the fact that it will continue collaborations with the William G. White, Jr. YMCA and Lowe's Foods, which are two well-known, regional organizations with long-standing reputations for community service. This study is designed as a three-arm randomized controlled trial. The three 12-month interventions include (1) a healthy aging control intervention that is based largely on educational sessions; (2) a standard physical activity intervention that involves structured, group-mediated physical activity sessions coupled with cognitive- behavioral counseling; and (3) the prosocial behavior physical activity intervention, which includes the components of the physical activity intervention plus the prosocial behavior incentive. The primary aims of the study are to compare the effects of the three interventions on 12-month levels of physical activity, physical function, and health-related quality of life. We will also assess whether these outcomes are mediated by changes in constructs from Social Cognitive Theory, and whether changes in Social Cognitive Theory constructs are associated with changes in prosocial behavior.